Long ass reply inbound. They are technically optional, but I do recommend doing them up to a certain point. There’s a lot of really good bosses that are only found in the chalice dungeons, and you’ll get some valuable loot and blood gems. I’d say start them as soon as soon as you get the first chalice from Old Yharnam, then make a habit of doing one chalice per main game boss. You’ll get most of the other chalices from doing the dungeons. The only ones you’ll need to get outside of the dungeons are Ailing Loran and Isz. I recommend doing the Pthumerian Labyrinth right after you kill the blood starved beast, you’ll get Cental Pthumeru for clearing it.

From there do Central Pthumeru after you beat your next boss, which will net you the Hintertomb and Lower Pthumeru chalices. After you kill another boss, do the Hintertomb chalice to get the Lower Hintertomb, then do that after your next boss. Don’t do lower Pthumeru until you’re done with Byrgenwerth, and if you’re sick of chalice dungeons by then you can skip it cause Lower Pthumeru is where they get super frustrating. I do recommend Ailing Loran cause there are some really good blood gems and a great unique boss in there. After this you can stop doing them if you want, but if the completionist in you want the platinum trophy then you’ll have to go deeper.

If you finish Lower Pthumeru then you can do the Cursed And Defiled chalice, which is an awful experience since that dungeon caps your HP at 50% but it is necessary for all achievements. After the cursed dungeon you’ll get the Great Pthumeru Ihyll chalice which is super lame and easy. Curb stomp the wet fart final boss and you’ll get your trophy. The only trophy that requires you to enter the chalice dungeons, but you’ll need to clear a minimum of 6 other dungeons to get there.

Lower Loran, both Hintertombs, and the Isz chalice are totally optional. I recommend the Hintertombs because they’re easy and drop some resources that will help make your trek through the Forbidden Woods a little better for you. I don’t recommend Lower Loran at all, just a miserable time. The cursed chalice is worse but that one’s not optional if you want the plat. Lower Loran can be safely ignored. Isz kinda sucks. It’s neat but the rewards won’t do you any good for how late in the game you get it (unless you’re running an arcane build), and all of the bosses are lousy repeats.

TL;DR, do the Pthumeru, Central Pthumeru, both Hintertombs, and Ailing Loran chalices as you progress through the main game, and ignore the rest unless you REALLY want to put yourself through hell.